1. Field of the Invention
The present invention relates to a rotary atomizing electrostatic coating apparatus for use in metallic paint coating.
2. Description of the Related Art
Japanese Patent Publication No. HEI 3-101858 discloses a rotary electrostatic coating apparatus using metallic paint. In the case where metallic paint containing aluminum or mica flakes is used, the speed at which the paint particles collide with an object to be coated is too low, resulting in a coated surface that is dark and without good brightness. To increase the collision speed shaping air is usually expelled at a high speed against the paint particles dispersed from an atomizing head to accelerate the paint particles in the direction toward the object to be coated. In this instance, the shaping air may be directed at an incline of about 30-40 degrees from a line parallel to an axis of rotation of the atomizing head to maintain good spreading despite using the high speed shaping air.
To obtain a high coating quality in metallic paint coating, the paint particles must collide with the surface of the object to be coated at a high speed. In a conventional coating, high pressure shaping air (for example, about 350-400 kPa) is expelled against the paint dispersed from the atomizing head so that the paint particles are accelerated toward the object to be coated. However, the shaping air expelled at a high pressure draws air around the shaping air flow to generate a secondary air flow accompanying the shaping air flow. As a result, when the shaping air flow reaches the object to be coated, the amount of air is generally increased to about 20-100 times more than the initial amount of the shaping air at the shaping air nozzles. Although the increased amount of air is necessary to carry paint particles to the object to be coated, the increased air also generates an air flow along the surface of the object to be coated, which prevents the paint particles from adhering smoothly to the surface of the object. This means that the use of high pressure air generates a considerably large amount of the air flow along the surface of the object so that the paint adhesion efficiency decreases, resulting in an increase in the consumption of the paint.
Further, the large amount of the air flow along the surface of the object whirls up paint particles which have not adhered to the object. As a result, the whirled-up paint particles adhere to the coating apparatus, the booth and the robot, and the adhering paint may drop onto the object to be coated to degrade or deteriorate the coating quality.